Many broad spectrum (white) LEDs in the market consist of blue or near UV light sources capped with a wavelength conversion layer that partially absorbs the short-wavelength light from the source and emits lower energy photons (e.g. green and red). The wavelength conversion layer may be a phosphor layer comprising Y3Al5O12:Ce3+ (YAG:Ce), which absorbs blue light (nominally 400-500 nm) and emits yellow light (nominally 550-600 nm).
Because of the typical planar configuration of the blue light source and the phosphor layers, the optical path of the blue emission across the converting layer is longer at shallow angles than at the normal direction (perpendicular to the plane of the die). As a result, the far-field spectral emission of the LED contains a higher fraction of blue photons at normal angles (more bluish) than at shallow angles (more yellowish). This effect is called “Color over Angle” (CoA), and is often referred to as the “yellow ring problem”.
Common Color over Angle correction techniques include increasing scattering of the phosphor layer and/or adding a scattering layer on top of the phosphor layer. For example, a highly scattering top layer (diffuser; ca. 50 microns thick) containing a dispersion of highly contrasting refractive index particles (titanium dioxide n˜2.4) in silicones (n˜1.5) may be used to “scramble” or reshuffle the directions of light emission. Scattering from nanoparticles is not truly isotropic, the nanoparticles scatter forward (away from the light source) more than backwards, and they also scatter more blue light than red light (lambda 4th power). Scattering from nanoparticles is also not angular-selective relative to the plane of the film. Also, in order for this layer to be effective to act as a diffuser, the optical path at shallow angles across the TiO2 layer is longer than the optical path at normal angles, and the mean free-path must be much shorter than the thickness of the film. This results in the inefficient “backscattering” of green and red light so that green and red light is absorbed by the phosphor layer.
In general, diffuser layers produce significantly lower device efficiency. Furthermore, the uniform dispersion of TiO2 nanoparticles requires specialized high shear mixing equipment that adds to the cost of manufacturing. In addition, several binder materials are solvent based (toluene, cyclohexanone), and are not amenable for use in high shear mixing systems (e.g. three roll mill).